576 busch. SUBSURFACE TECHNIQUES [Ch. 31 



of a profile will be mapped as a lense, whereas often a more judicious 

 selection of wells would prove this sand to be a wedge with its elongate 

 direction parallel to the shore. Unconformities are nearly impossible 

 to recognize from dog-legged profiles owing to the apparent lack of 

 systematic change in sedimentary sequence. 



LITHOFACIES 



When all wells have been "tied" in to a lattice control, it is a com- 

 paratively easy task to identify and outline known pools of oil and 

 gas by individual sandstone or limestone formations. A direct com- 

 parison of the positions of oil and gas pools with the variations in thick- 

 ness of the formation in which they occur sometimes makes it possible 

 to relate the incidence of these hydrocarbons to a certain range in 

 thickness. In addition to thickness maps, Krumbein (1945, pp. 1254- 

 1259) has outlined numerous other contour-type sedimentary maps 

 which can be constructed to show attributes of sedimentary rocks, 

 some of which might bear apparent relationship to the occurrence of oil 

 and gas. In a later paper Krumbein (1948, p. 1910) emphasizes the 

 desirability "of contrasting the clastic components and non-elastics in 

 the section" being investigated. He points out that 



A first approximation to the over-all lithological character of a measured 

 outcrop of subsurface section may be had by grouping the rocks into elastics 

 (conglomerate, sandstone, shale) and non-elastics (limestone, dolomite, evap- 

 orite) on the usual conventional basis. The thicknesses (or percentages) of 

 elastics are added together and divided by the sum of the thicknesses (or per- 

 centages) of the non-elastics. The resulting number is defined as the "clastic 

 ratio." The clastic ratio is augmented by a "sand-shale ratio," which is the 

 ratio of sandstone (plus conglomerate) to shale in the section, regardless of 

 the amount of non-elastics present: 



_,, . . conglomerate + sandstone + shale 



Clastic ratio = — ; — :— i -. — ; — 



limestone + dolomite + evaporite 



_, . . , . conglomerate + sandstone 



Sand-shale ratio = : — : 



shale 



These ratios may be readily visualized by considering them as indices of 

 the relative amounts of material in the numerator of the ratio deposited per 

 unit thickness of material in the denominator. 



Besides isopach, clastic ratio, and sand-shale ratio maps, several 

 additional types of lithof acies maps are of considerable value in a study 

 of sedimentary rocks. They include 



